1. Field of the Invention
The present invention relates to a method for preparing alkylnaphthalenes, and more particularly to a method wherein naphthalene containing thianaphthene as an impurity is alkylated with a lower olefin to obtain alkylnaphthalenes containing monoalkylnaphthalenes with a predominant amount of a .beta.-monoalkylnaphthalene in a specific solvent which dissolves the naphthalene and has a boiling point sufficiently different that the solvent is separable from naphthalene and the resulting alkylnaphthalenes by distillation, using, as a catalyst, aluminum chloride complexes or a combination of solid aluminum chloride and hydrogen chloride gas which is added to the reaction system in the amount required to dissolve the aluminum chloride.
2. Description of the Prior Art
As is well known, alkylnaphthalenes are very important intermediates, irrespective of the two isomeric .alpha. and .beta. forms, in various industrial chemical fields, for example, in the fields of dyestuffs, medicines, agricultural chemicals, synthetic resins, lubricating oils and surface active agents, and applications are now being expanded. Among alkylnaphthalenes, as .beta.-monoalkylnaphthalene has recently attracting special interest as a material for .beta.-naphthol and the like.
Alkylnaphthalenes are commonly prepared by alkylation of naphthalene with olefins, and a great deal of study has so far been made on the alkylation, particularly on alkylation catalysts. Catalysts such as sulfuric acid, phosphoric acid, hydrogen fluoride, boron trifluoride and aluminum chloride are now in practical use.
These catalysts, however, are not always satisfactory. For example, hydrogen fluoride and boron trifluoride are highly corrosive to the equipment irrespective of their extremely high catalytic activity, and sulfuric acid and phosphoric acid have a low catalytic activity although they are very easy to handle. On the other hand, aluminum chloride is very advantageous as a practical catalyst. Aluminium chloride has a catalytic activity which is not as high as hydrogen fluoride and boron trifluoride but which is higher than sulfuric acid and phosphoric acid, has a relatively mild corrosiveness, and is relatively easy to handle. Furthermore, a larger proportion of .beta.-alkylnaphthalenes in the resulting alkylnaphthalenes can be obtained using aluminum chloride catalysts than using hydrogen fluoride, boron trifluoride, sulfuric acid and phosphoric acid, which is very advantageous for an object of the present invention.
However, aluminum chloride catalysts have the disadvantages that the catalytic activity is remarkably reduced by impurities in the naphthalene starting material whereby the reaction progress becomes very difficult and a great increase in the amount of catalyst used becomes necessary. Therefore, the impurities must be removed from the naphthalene by purification which is very troublesome and uneconomical.
As is well known, naphthalene as an industrial material is produced somewhat differently from other typical aromatic compounds such as benzene, toluene and xylene. That is, with the latter compounds referred to as BTX, even those which are available as an industrial material have a quite high purity in most cases, whereas naphthalene available as such is mostly a by-product produced from a manufacturing process of the iron and steel industry, and commonly has a low purity as is indicated by the name "crude naphthalene".
The composition of crude naphthalene is not always definite, and in most cases the naphthalene content is about 95 to 97%. Impurities present in the naphthalene include thianaphthene, .alpha.-methylnaphthalene, .beta.-methylnaphthalene, quinoline and high boiling materials referred to as tar. In particular, approximately 90% of the impurities is thianaphthene although the content varies somewhat with crude naphthalenes. Therefore, most of the impurities in the crude naphthalene can be regarded as thianaphthene, and the reduction in activity of aluminum chloride catalysts may be primarily due to the thianaphthene.